1. Technical Field
This invention relates generally to an integral rigid chip carrier board and cable assembly and more particularly to such an assembly having selectively disposed tear-resistant reinforcement members.
2. Background Art
Multilayered rigid printed circuit boards having flexible printed circuit cables extending from the rigid board are particularly desirable in electronic components having high density circuit requirements. One example of such a circuit is disclosed in U.S. Pat. No. 4,800,461 issued Jan. 24, 1989 to Dixon, et al. The Dixon patent describes a multi layer rigid-flex printed circuit structure using insulator materials which, when subjected to elevated temperatures, do not expand sufficiently to cause damage to the structure.
More recently, a multilayer structure, identified as a high performance carrier, is formed of a plurality of decrete wiring cores that are selectively laminated to form an integral structure having a multilayer rigid-card region and flexible cables extending outwardly from the rigid-card region. The materials used to construct the individual core and cable subassemblies are silica (SiO.sub.2) filled polytetrafluoroethylene (PTFE), normally having a thickness of about 50 .mu.m thick when fully densified, a 25 .mu.m thick layer of copper-invar-copper (CIC) and copper having a thickness of about 5-12 .mu.m for the conductive layers. The overall final thickness of the individual core-cable subassembly ranges from about 250 .mu.m to about 275 .mu.m.
The materials selected for construction of the high performance carrier are selected to provide optimum electrical signal transmission and insulation properties. However, it has been found that the high performance carrier is prone to transverse tearing, particularly in the junction area between the rigid card section and the flexible cable section of the assembly. Once a tear has been initiated, and propagation started, only a very small force is necessary to extend the fracture across the structure. This characteristic of the structure reduces potential reliability and structural integrity. The addition of additional reinforcement layers throughout the structure, or the use of thicker dielectric materials, is undesirable because of the added thickness that would result in the structure, the added material costs that would be incurred, and the added manufacturing steps that would be required to form a fully reinforced structure.
The present invention is directed to overcoming the problems set forth above. It is desirable to have a multilayered structure, having a rigid chip carrier board and integrally formed flexible cables extending from the chip carrier board, that is resistant to transverse tearing. It is also desirable to have such a structure that is economical to produce and does not significantly add to the overall thickness of the structure.